Structural and Selective Gas Adsorption Studies of Polyoxometalate and Tris(ethylenediamine) Cobalt(III) Based Ionic Crystals

Abstract

Seven new ionic solids built on polyoxometalate anions and [Co(en)₃]³⁺ cations, namely, [Co(en)₃Mo₈O₂₆(H₃O)(Cl)](DMF)₄(H₂O) (Co-Mo₈-DMF), [α-PW₁₂O₄₀][Co(en)₃]·6DMF (Co-W₁₂P-DMF), [α-PW₁₂O40][Co(en)₃]·6DEF (Co-W12P-DEF), [α-PMo12O40][Co(en)₃]·5.5DMF (Co-Mo12P-DMF), [α-PMo12O40][Co(en)₃]·6DEF (Co-Mo12P-DEF), [α-SiW12O40][Co(en)₃]3/2[Cl]1/2·6DMF·3H2O (Co-W12Si-DMF), and [α-SiW12O40][Co(en)₃]·6DEF (Co-W12Si-DEF), have been synthesized from nonaqueous (DMF/DEF) media and characterized by single-crystal X-ray diffraction. We attempt to understand if [Co(en)₃]³⁺ cations used in these reaction systems play a crucial role in controlling the assembly of these crystals. These ionic solids, after removal of the DMF or DEF molecules, are found to exhibit size selective H₂ adsorption properties over N2. The amount of hydrogen adsorption was influenced by POM anion types and their orientations. CoMo₁₂P-DEF has the highest (0.9 wt %) H₂ uptake, and CoW₁₂P-DEF has the lowest (0.4 wt %) uptake among the series when the adsorbate pressure approached 1 atm.